Interesting focus question. Please Help!

I just had a look at the link and the explanation makes perfect sense to me.

He says "...it means that all objects lying on a plane parallel to and 6 feet away from the film/sensor plane will be in focus"

Doesn't this mean that the vertical red line is wrong? It should be perpendicular to the red line that is coming out of the camera and indicates that the plane of focus will intersect the models face at a single place.

Sorry, my training in 3D modeling is coming through.

Rob

BiPolarBear wrote in post #4251003
I may way wrong, but if you use "focus lock" when you focus on the eyes (as per the example), even when you lower the aim and recompose, the focus is still locked at where you locked it and the eyes will still be in focus.
Assuming you don't accidently move the camera closer/further away by swaying a bit of course.
Or am i just fundamentally lost?
Greg

The focus doesn't remain fixed on the eye, it remains fixed on the DISTANCE the eye was from the sensor. This is where I get confused, if the plane of focus is curved then the eye would be in focus, if however the plane of focus is parallel to the sensor the close up at shallow DOF the eye could be out of focus. Mind you unless you're close up wide open with a fast lens we're verging on the theoritical.

hastur wrote in post #4251385
I just had a look at the link and the explanation makes perfect sense to me.

He says "...it means that all objects lying on a plane parallel to and 6 feet away from the film/sensor plane will be in focus"

Doesn't this mean that the vertical red line is wrong? It should be perpendicular to the red line that is coming out of the camera and indicates that the plane of focus will intersect the models face at a single place.

Sorry, my training in 3D modeling is coming through.

Rob

Ummm - not quite, sorry.
"...when the camera is tilted back to achieve correct framing, the center of focus will be along the vertical red line."

The red line represents the recomposed composition. Basically if you leave the camera tilted up, the focus will be parralel with the sensor plane. and the line would look different. His example is saying that you get your focus on the eyes at parralel to the sensor and then recompose so the parralel is vertical with the composition. When you recompose from that focus distance, the parralel line will be further away than if you focus from that plane in the first place.

EOS MAN1 wrote in post #4250057
If you focus on the eyes and then recompose, won't you get out of focus eyes because the distances differ. ...... I am having problems with nailing the focus on the eyes, could this be why?

As others have noted, focusing and recomposing can be a problem when you're close to the subject and/or using a very narrow DOF. To test this, try shooting from the same distance without recomposing and see if you're getting the proper focus you would expect. If not, its possible that your XT or lens needs to be calibrated (i.e., you're getting front- or back-focusing). Shooting some focusing charts can help figure out if this is the case. Its also possible that you're just finding the XT has less precision auto-focuising with fast lenses than you're expecting -- this is exactly why I upgraded from the XT to the XTi.

EOS MAN1 wrote in post #4250057
If you focus on the eyes and then recompose, won't you get out of focus eyes because the distances differ. These lines make a triangle. The Hypotenuse(eyes) is always the longest line in a triangle. I am having problems with nailing the focus on the eyes, could this be why?

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I have definantly been noticing the effects of this with f1.4-f2.8 lateley with a 50 1.4 - being more selective in my focus point now seeing as i will often shoot my son very close up...

also i notice in your diagram you must be using a 40D or MKIII with live view and shooting from the chest. What lens is that

BiPolarBear wrote in post #4251003
I may way wrong, but if you use "focus lock" when you focus on the eyes (as per the example), even when you lower the aim and recompose, the focus is still locked at where you locked it and the eyes will still be in focus.
Assuming you don't accidently move the camera closer/further away by swaying a bit of course.
Or am i just fundamentally lost?
Greg

When you use "focus lock" it just locks the lens at the currently selected focal "distance". The lens has no way of knowing you are aiming it at eyes or any other particular object. The focusing mechanisms determine how the lens should be set to focus on a given object. If you lock in your distance setting and then point the lens at something else the distance setting remains the same.

The act of recomposing will move the focal plane to some extent relative to the original focal point. How much would depend on the distance from lens to object as noted by TomC723.

TomC723 wrote in post #4250791
Unless you're shooting with a wide aperture (2 or 1.8 for example), I don't think you have much to worry about. With a little math, we can figure out the distance change. Say we're shooting from 8ft away. And we'll use me as the sample subject (I'm 5'5"). The distance from my eyes to just below my sternum is 15 in. Using the Pythagorean theorem, we can calculate the distance of the hypotenuse as sqrt(15^2 + 96^2) and get a distance of about 97 inches.... a very minuscule change. And the farther away you shoot, the smaller the distance change... So unless you're shooting fairly close at a wide aperture, I wouldn't worry about it too much as your DOF should cover that small distance.

While your math is right, using http://www.dofmaster.c​om/dofjs.html you can calculate that the DOF with a 100mm F2 @ 8ft away is only 1.68 inches.

Of course, you should also consider that the vertical FOV for a vertical portrait taken at 100mm 8ft away is about 20inches. Logic tells us that this equals about 10 inches on either side of the viewfinder...So, if you center focus on a persons eyes, and then move down 15 inches to their sternum ( as drawn ) their eyes wont even be in the picture anymore.


To sum it all up, the ideal way to take a picture is to focus and then release the shutter. Recomposing is moving the distance and orientation of your DOF with relation to your subject.
That being said, I always focus/recompose and never have a problem.

BiPolarBear wrote in post #4251003
I may way wrong, but if you use "focus lock" when you focus on the eyes (as per the example), even when you lower the aim and recompose, the focus is still locked at where you locked it and the eyes will still be in focus.
Assuming you don't accidently move the camera closer/further away by swaying a bit of course.
Or am i just fundamentally lost?
Greg

Fundamentally lost. The camera doesnt focus on an object. It more literally focuses at a distance. Your depth of field is a perpendicular plane located X distance away from the lens where X is the length the camera focused at. If you focus on eyes 97" away, they will ( ideally... ) be in the middle of that imaginary plane. By moving the camera to the sternum, objects ( including the eyes ) are now 96" away from the lens ( Its 96 because the effective distance when talking about focus is only perpendicular distance since DOF's cant curve ).

So, by doing that youve shifted the eyes backward and potentially out of your originally focused DOF.


Sorry if its hard to understand. I dont explain things very well in person and do even worse on the net.